1. Field of the Invention
The present invention relates to a double tunnel junction structure used as a tunnel junction sensor in a magnetic head, and more particularly, to a double tunnel junction structure having enhancement layers that boost the magnetoresistance with multiple barriers used to eliminate the effect of the applied dc bias without reduction in spin polarized tunneling.
2. Description of the Related Art
A read head employing a read sensor may be combined with an inductive write head to form a combined magnetic head. In a magnetic disk drive, an air bearing surface (ABS) of the combined magnetic head is supported adjacent a rotating disk to write information on or read information from the disk. Information is written to the rotating disk by magnetic fields which fringe across a gap between the first and second pole pieces of the write head. In a read mode, the resistance of the read sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a current is conducted through the read sensor, resistance changes cause potential changes that are detected and processed as playback signals.
A read sensor is employed by a magnetic head for sensing magnetic fields from moving magnetic media, such as a magnetic disk or a magnetic tape. One type of read sensor employs a tunnel junction sensor. The typical tunnel junction sensor includes a nonmagnetic spacer layer sandwiched between first and second ferromagnetic layers, commonly called a pinned layer, and a free layer. The magnetization of the pinned layer is pinned 90xc2x0 to the magnetization of the free layer and the magnetization of the free layer is free to respond to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic pinning layer.
The tunnel junction sensor is based on the phenomenon of spin-polarized electron tunneling. The typical tunnel junction sensor uses ferromagnetic metal electrodes, such as NiFe or CoFe, having high coercivity with a spacer layer that is thin enough that quantum mechanical tunneling occurs between the ferromagnetic layers (FM/I/FM). The tunneling phenomenon is electron spin dependent, making the magnetic response of the tunnel junction sensor a function of the relative orientations and spin polarization of the two ferromagnetic layers. The details of tunnel junction structures have been described in the commonly assigned U.S. Pat. No. 5,650,958 to Gallagher et al., which is incorporated by reference herein.
FIG. 1 shows tunnel magnetoresistance (TMR) as a function of dc bias for a tunnel junction sensor. At low dc bias, the conduction varies only slightly with the dc bias. As the dc bias increases, the TMR coefficient drops noticeably. For example, the application of 300 mV bias across a tunnel junction structure having a structure comprising ferromagnetic/insulator/ferromagnetic (FM/I/FM) reduces the TMR by half.
To solve this problem, another type of tunnel junction sensor has been proposed called a double junction sensor (FM/I/FM/I/FM). FIG. 2 shows a prior art tunnel junction sensor 200 which includes a first pinning layer 205, a first pinned layer 210, a first spacer layer 215, a free layer 220, a second spacer layer 225, a second pinned layer 230 and a second pinning layer 235. The magnetization of the outer two FM pinned layers are parallel while the magnetization of the internal FM free layer is either parallel or antiparallel. Modeling has shown that the double tunnel junction behaves differently than the traditional single tunnel junction by eliminating the effect of dc bias. FIG. 3 shows the TMR as a function of the dc bias for a double junction tunnel junction sensor.
While it appears that the multiple barriers have been shown to significantly eliminate the effect of dc bias, the double tunnel junction has drawbacks. For the spin polarized resonant tunneling phenomenon to work, the layers of the double tunnel junction must be made very thin. While it is desired to have thin layers, too thin a layer is detrimental to the device. For example, the center FM layer (traditionally the free layer) for the prior art is between 10 and 20 xc3x85. With a layer this thin, the ferromagnetic free layer becomes saturated easily from external magnetic fields. Once saturated, the double tunnel junction sensor does not get the full benefit of the ferromagnetic free layer, the signals get clipped. It is preferable that the free layer never be saturated.
From the above discussion it becomes apparent that what is needed is a double tunnel junction sensor that provides the benefits of improved spin polarized tunneling and minimizing dc bias effects while also providing a device in which the internal layers are not saturated by an external magnetic field.
The present invention is directed toward an enhanced double tunnel junction structure that has enhancement layers causing resonant tunneling which boosts the magnetoresistance (MR), achieving higher tunnel magnetoresistance (TMR) for the structure. This is accomplished by using enhancement layers that create a quantum well between the enhancement layer and the pinned layer. By doing this, the tunneling constraints on the free layer are decoupled, allowing the free layer to be made thicker ( greater than 20 xc3x85) and reducing or eliminating saturation from an external magnetic source.
In one embodiment, the resonant enhanced double tunnel junction sensor includes a first shield, a first pinning layer, a first pinned layer, a first enhancement layer, a first spacer layer, a free layer, a second spacer layer, a second enhancement layer, a second pinned layer, a second pinning layer and a second shield layer. In the preferred embodiment, the enhancement layer is made form copper (Cu). In another embodiment, the free layer is a multi-layered material having 75% NiFe and 25% Co90Fe10.
In the preferred embodiment, the magnetic moment of the first and second pinned layers are pinned by interfacial exchange with the magnetic spins of the first and second pinning layers in a downward direction, perpendicular to the ABS, while the magnetic moment of the free layer is perpendicular to the magnetic moment of the first and second pinned layers (i.e., the moment direction being parallel to the ABS). In use, a tunneling current IT, using spin dependent electron tunneling, flows through the enhanced double tunnel junction sensor, using the first and second shield layers as leads. The amount of current IT that flows through is dependent on the relative magnetic moment directions between the first and second pinned layers and the free layer. In prior art double tunnel junctions, the free layer must be thin to perform properly and is prone to become saturated quickly from the external magnetic field. To solve this problem, the present invention adds enhancement layers of copper (Cu) to boost the magnetoresistivity (MR) of the sensor. The copper enhancement layers increase the spin polarized resonant tunneling, giving the structure a high TMR. With the higher TMR, the free layer may be made thicker and not saturate as easily. As the enhanced double tunnel junction sensor is positioned over the magnetic disk, the external magnetic fields sensed from the rotating disk moves the direction of magnetic moment of the free layer up or down, changing the resistance through the tunnel junction sensor. As the tunnel current IT is conducted through the sensor, the increase and decrease of electron tunneling (i.e., increase and decrease in resistance) are manifested as potential changes. These potential changes are then processed as readback signals by the processing circuitry.
Another embodiment of the present invention is an antiparallel (AP) resonant enhanced double tunnel junction sensor. This AP double tunnel junction sensor is similar to the double tunnel junction sensor described above but utilizes first and second AP pinned layers and in place of the first and second pinned layers. The AP pinned layer consists of a spacer made of ruthenium (Ru) between pinned film layers, preferably made of cobalt (Co). Because of the antiparallel features of the AP layers due to the Ru spacer layer, the magnetic moment of the one pinned film is antiparallel to magnetic moment of the other pinned film, which increases the effect of the sensor when the magnetic moment of the free layer rotates. In other embodiments, a combinations of pinned and AP pinned layers are used.
Other objects and advantages of the present invention will become apparent upon reading the following description taken together with the accompanying drawings.